About the Soffit Calculator
The soffit is the underside of the roof overhang, the horizontal surface between the outside wall and the fascia board at the edge of the roof. It closes the eaves against birds and insects and gives the house a finished look. On most homes it also does an important job: vented soffit panels let outside air into the attic, where it flows up under the roof deck and out through ridge or roof vents, keeping the attic dry in winter and cooler in summer.
This soffit calculator works out how many panels you need for your eaves, how much J-channel or F-channel trim to hold them, and, if you enter your attic floor area, how much vented soffit is needed for enough intake air under the common 1/300 ventilation rule.
How to Use the Soffit Calculator
Enter the total eave length: add up every run of eaves that will get soffit.
Enter the soffit depth, measured from the wall to the back of the fascia, in inches.
Choose the panel coverage, enter a waste allowance, and optionally the attic floor area and the net free area of the vented soffit you plan to use.
The Formulas
soffit area = eave length × depth ÷ 12
panels = ⌈area × (1 + waste) ÷ panel coverage⌉
trim pieces = ⌈eave length × 2 × 1.1 ÷ 12⌉
intake needed = attic floor area ÷ 300 ÷ 2 × 144 (in²)
vented soffit = intake ÷ net free area per ft²Step-by-Step Example
120 ft of eaves, 16 inches deep, 12 ft² panels, 10% waste, 1,500 ft² attic, 9 in²/ft² vented soffit.
area 120 × 16 ÷ 12 = 160 ft²
panels 160 × 1.1 ÷ 12 = 14.7 → 15 panels
trim 120 × 2 × 1.1 ÷ 12 = 22 pieces of 12 ft
intake 1,500 ÷ 300 = 5 ft² total, half = 2.5 ft² = 360 in²
vented 360 ÷ 9 = 40 ft², a quarter of the soffitThe 1/300 Ventilation Rule
Building codes commonly require at least 1 square foot of net free ventilation area for every 300 square feet of attic floor, provided the vents are balanced between the low part of the roof, at the soffits, and the high part, at the ridge or roof vents. Without that balance the rule becomes 1/150, twice as much. Splitting it evenly, half the area goes at the soffits as intake. Some experts prefer slightly more intake than exhaust, so the attic is not depressurised and does not draw conditioned air from the house.
Net Free Area
A vented soffit panel is not all holes. Net free area is the open area that air can actually pass through, after the louvres, perforations and insect screen. Manufacturers print it on the packaging, often 6 to 10 square inches per square foot for vinyl and aluminium. Using the gross panel area instead would overstate the ventilation many times over. If the attic needs more intake than your soffit can provide, use a panel with a higher net free area, or add continuous strip vents.
Materials and Panel Sizes
Soffit comes in vinyl, aluminium, steel, fibre cement and wood. Vinyl and aluminium panels are usually 12 or 16 inches wide and 12 feet long, cut to the soffit depth and installed across it. Each 12-foot panel covers 12 or 16 square feet depending on width. For deep overhangs, one panel length may cover several runs of soffit, so the waste allowance covers the offcuts. Solid panels are used where no ventilation is needed, such as porch ceilings.
Trim and Installation
Panels are usually held in J-channel against the wall and in F-channel or J-channel at the fascia, then covered with fascia wrap. Allow two lengths of channel per eave run plus extra for corners and laps. Panels need a small gap in the channels for expansion, and nailing should not be tight, so vinyl can move with temperature.
Signs of Poor Attic Ventilation
Without enough intake air, attics trap heat and moisture. In winter, warm, damp air from the house condenses on the cold underside of the roof deck, leading to mould, rot and stained ceilings. In cold climates, a warm roof melts snow that refreezes at the eaves, forming ice dams that push water under shingles. In summer, a hot attic overheats rooms below and shortens shingle life. Frost on nail tips, damp insulation and peeling paint on the soffit are clues that the attic needs more ventilation.
Measuring the Eaves
Measure along the fascia of each eave, and include the short returns at gable ends if they get soffit. Measure the depth at several points, because overhangs often vary slightly around a house. For a hip roof, every side has eaves, so the eave length is roughly the perimeter of the house plus the overhangs at the corners. For a gable roof, only the two long sides usually have vented soffit, though the rake overhangs may be enclosed with solid panels.
Understanding Your Result
The headline gives the number of soffit panels.
The area line shows the soffit square footage and waste.
The trim line gives the J- or F-channel pieces.
The ventilation line gives the intake area needed and the share of vented soffit.
When Should You Use This Calculator?
Use it when replacing or adding soffit during re-siding or re-roofing.
Use it to check whether an attic has enough intake ventilation.
Use it to order panels and trim together.
Common Mistakes
Using gross panel area for ventilation. Use the net free area.
Blocking vents with insulation. Fit rafter baffles.
Forgetting gable-end runs. Add up every eave that gets soffit.
Nailing vinyl tight. Leave room for expansion.